Revision as of 09:32, 18 July 2012

Contents

Edinburgh Buses: self join

A table may be joined with itself. When this happens we need
some mechanism for distinguishing the instances of the table.
Labels may be introduced in the FROM clause - a dot is used to
separate the label and attribute name when used elsewhere in
the statement.

Examples of self joins on the route table

Self join of route on (num, company)

We might join the route table on the (num, company) pair.
The result is a list of all pairs of stops which share a
service.

This is a large table with over 11000 entries. An extract is
shown below. There are 121 entries the LRT 1 service alone.
There are 11 stops on the number 1. It is a circular route -
the first and last stops are identical.

num

company

pos

stop

num_1

company_1

pos_1

stop_1

1

LRT

1

134

1

LRT

7

217

1

LRT

1

134

1

LRT

11

134

1

LRT

1

134

1

LRT

10

79

...

1

LRT

2

97

1

LRT

7

217

1

LRT

2

97

1

LRT

11

134

...

2

LRT

9

31

2

LRT

11

217

2

LRT

9

31

2

LRT

1

168

...

Self join of route on stop

We might join the route table with itself on the stop field.
The result is a list of all pairs of services which share a
stop.

SELECT * FROM route R1, route R2
WHERE R1.stop=R2.stop;

In the extract below some of the entries for stop 53,
'Craiglockhart' are shown.